JPS6179286A - Laser diode and mode hopping prevention therefor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a raster output scanner comprising a laser diode and a holographic beam scanning device, and more particularly to:
A laser diode improved to prevent mode hopping for use in a raster output scanner, and a method of preventing mode hopping in such a laser diode.

BACKGROUND OF THE INVENTION As the technology of raster output scanners (referred to as RO3) has evolved, one major improvement has been the use of scanning polygons and their typically associated relatively complex cylindrical corrective optics. was replaced by a holographic scanning device. In this type of scanning device, a holographic disk with successive diffractive facets around its periphery is used instead of a polygon, and the imaging beam is raster scanned across the recording member. Another comparable improvement has been the replacement of relatively large gas lasers and their associated modulators with relatively small internally modulated laser diodes.

However, when laser diodes are combined with holographic scanning devices, there is at least one significant problem: the tendency of the diodes to hop due to the frequency instability inherent in laser diodes. This tendency in laser diodes is due to their extremely broad gain curves and their tendency to change frequency with temperature changes. As a result, the laser diode typically operates as a single longitudinal mode laser, but changes from one longitudinal mode to another as temperature or equivalent duty cycle or power level changes.

Numerous methods have been used in the past to stabilize laser diodes, limit their operation to a single mode, and prevent mode hopping, but none have been commercially successful or practical. There are almost no For example, one method uses etalons, birefringent filters, or blazed concave gratings to prevent mode hopping. However, these types of devices are extremely expensive and therefore unattractive, especially when cost is a consideration. Other laser diode stabilization methods using external cavities use simple end mirrors. However, to be successful, this method requires extremely high mechanical stability, which is difficult to achieve in raster scanners that have many moving parts, including relatively large recording members. It is difficult if not possible. This latter method also does not allow for frequency tuning, which is a particularly advantageous property when using holographic scanning devices.

Referring to a conventional raster output scanner (RO5) shown in FIG. 1, a high intensity laser beam 10 modulated according to an image signal input is scanned across a recording member 12 to selectively expose the member 12. The image created on recording member 12 is then developed into a visible image.

Recording member 12 comprises any suitable recording medium, such as a photoreceptor in a xerographic imaging device. In this type of device, the photoreceptor is first uniformly charged and then exposed to beam 10 to form an electrostatic latent image on the photoreceptor represented by the image signal input. after that,
The electrostatic latent image is developed and the developed image is transferred to a suitable copy substrate material such as paper. The image is then fixed to become a permanent image. The recording member 12 is
It may be of any suitable type, such as a drum, web, belt, etc.

The beam 10 is generated by a suitable laser diode 15, for example a Hitachi 7801E diode.

An internal modulation circuit allows the beam output by diode 15 to be modulated by the image signal input. The image signal may be obtained from any suitable source, such as a memory, an input scanner, a communication channel, etc.

Modulated beam 10 output by diode 15
passes through a suitable reference lens 20 and a compensating grating 27 and impinges on the grating facet 24 of the rotating holographic disc 22 at an angle of approximately 45°. The disk 22 has facets 24 following one after the other around its periphery, so that when the disk 22 is rotated by the motor 25, each facet 24 causes the beam 10 to scan across the recording member 12 in the X direction. At the same time, the recording member 12
is driven in the Y direction (first direction) by suitable driving means (not shown).
(in the direction shown by the solid arrow in the figure). A suitable lens 23 is provided downstream of the disk 22 to focus the beam 10 to a point on the recording member 12.

In the apparatus of FIG. 1, a frequency-compensated diffraction grating 27 was first used to allow a wide selection of laser diodes operating at different wavelengths. However, the diffraction grating 27 also
Disable mode hopping effects in the horizontal scan direction at all scan angles and fast scan effects only at zero scan angle.

Diffraction grating 27 is arranged in a plane parallel to the plane of rotation of holographic ink disk 22, and beam 10 is incident on diffraction grating 27 at an angle θ of about 456. As a result, the output beam is directed to the diffraction grating 2 of the holographic disk 22.
7 and facet 24 at an angle of approximately 45°. However, as mentioned above, grating 27 only provides wavelength compensation at zero scan angle. The result is mode-hopping induced jitter along the scan axis (ie, the X-axis), the magnitude of which is determined by the operating parameters of the raster output scanner.

(Problems to be Solved by the Invention) The present invention seeks to correct the above-mentioned conventional problems and provide a relatively inexpensive and practical single mode laser diode for use in a raster scanner having a bolographic scanning device. It is something to do.

The present invention also seeks to provide a method for preventing mode-hole repping in a laser diode to enhance its use as an image scanning beam source in a raster output scanner.

Means for Solving the Problems The laser diode of the present invention includes an anti-reflection coating on the front facet of the diode internal cavity and an anti-reflection coating for collimating the light emitted from the laser diode front facet into a parallel bundle of beams. a reference objective lens, a holographic ink planar diffraction grating for dispersing said beams by wavelength; and reflecting only one of said beams onto itself and through said grating and reference lens into a diode. a beam splitting mirror adapted to return the beam, the lens focusing the one beam onto the front facet of the laser diode to cause the laser diode to operate at the wavelength of the one reflected beam.

The method includes the steps of coating the front lasing facet of a laser diode with an anti-reflection material, collimating the light emitted by the laser diode into a parallel beam, and dispersing the beam according to its wavelength. and reflecting said beam of one wavelength onto itself and back into said laser diode."
- operating the laser diode in a single mode at the wavelength of the reflected beam.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

EXAMPLE To further describe the invention with reference to FIG. 2, a laser diode 15' is modified to change the diode from an internal cavity type to an external cavity type, with
8 is used only as a gain medium. This corresponds to the diode's internal cavity beam-emitting facet 34 (
The reflective surface of the front facet is made of silicon monoxide (Sin).
This is done by coating with a suitable anti-reflection coating 36 such as. This significantly increases the losses within the diode internal cavity 38, so that at normal operating currents the gain is much less than the losses.

As a result, the internal cavity threshold current is significantly higher than the threshold current when lasing previously occurred within the internal cavity. However, the external cavity threshold currents described herein are significantly lower than the raised internal cavity thresholds discussed above. Therefore, at normal operating currents, the external cavity predominates over the internal cavity. The back facet 33 of the diode can be coated with a reflective coating 37 to reduce losses in the external cavity.

An external cavity 39 is formed in the path of the beam 10 output by the diode 15' with a suitable reference objective lens 40 to collimate the light of different wavelengths (resulting from laser mode hopping) emitted by the diode 15'. Reference is made to the beam 44 of the bundle.

A diffraction grating 46 directs the light to an appropriate beam splitting mirror 5 according to the wavelength of the individual rays 44 making up the beam.
Distribute over 0. Mirror 50 is positioned so that only one ray 44' at a preselected wavelength is reflected by mirror 50 onto the ray and through diffraction clump 46 back to reference lens 40. The lens 40 reflects light 44
′ on the front facet 34 of the internal cavity 38;
Diode 15' is operated in a single mode at a preselected wavelength. Rays 4.4 A and 44 B are reflected back through this optics but are not focused back into the gain medium. As a result, the operation of diode 15' is stable and mode hopping is eliminated.

Diffraction grating 46 or mirror 50 or diffraction grating 46
External cavity 39 can be tuned by movement, such as by rotating both mirror 50 and mirror 50 together, to select the desired optimal operating mode and wavelength.

Referring to the embodiment shown in FIG. 3, like reference numerals indicate like parts, there is provided a housing 60, a laser diode 15', a reference lens 40, a diffraction grating 46,
and a beam splitting mirror 50 are supported as a unit in a predetermined working relationship with each other. In this embodiment, a beam mirror 50 is placed between prisms 62 and 62' and a single mode output beam 44'
The holographic disk 22 along the system optical axis
is reflected to the facet 24 of.

In the embodiment shown in FIG. 4, like reference numerals indicate like parts, and for the beam, mirror 50 is replaced by a total reflection mirror 65. As a result, the output beam 10 is extracted from the zero order diffraction grating 46. Since the diffraction grating 46 cannot be made to be 100% efficient, some amount of zero order light is always present. Ordinary diffraction gratings have an efficiency of about 7
0% to 80%, so about 20% to 30% of the energy is in the zero order beam and is therefore available for output. Therefore, substantially all of the laser energy is used for return or output.

In the embodiment shown in FIG. 5, like reference numerals indicate like parts, mirror 50 is omitted for the beam portion, and holographic grating 46 is replaced by a reflective grating 70, which is used to draw the beam. operating in the configuration to act as a frequency selective return and beam splitting member. Diffraction grating 70 diffracts approximately 70% of the energy at the selected wavelength back into diode cavity 38 as first order diffraction, leaving approximately 30% of the energy as zero order diffraction for output beam 10. As a result, this embodiment provides a high net return with little wasted energy.

Although the present invention has been described above with reference to embodiments thereof, the present invention is not limited thereto, and various modifications and changes can be made within the scope of the present invention as set forth in the claims.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram illustrating a conventional raster output scanning device with means for nullifying the fast scan effects of laser diode mode hopping only at zero scan angle, and in which the transverse scan effects are nullified at all scan angles; FIG. 2 is a schematic diagram illustrating an apparatus for preventing laser diode mode hopping of the present invention, using the diode internal cavity only as a gain medium and providing an external lasing cavity with means for limiting diode operation to a single longitudinal mode; FIG. , FIG. 3 is a partial cross-sectional view of the mode-hopping prevention device of the present invention incorporated into a unitary structure for interaction with a holographic scanning device, and FIG. 4 shows that the beam portion of FIG. 2 is replaced by a mirror. Figure 5 shows a second alternative embodiment using a reflective grating in place of the beam splitting mirror and transparent grating of Figure 2; This is a schematic diagram. 33... Back facet, 34... Front facet, 36... Antireflection coating, 37... Reflective coating, 4
0... Reference lens, 46... Holographic diffraction grating, 50... Beam portion is mirror, 62.62'...
- Prism, 65... Total reflection mirror, 70... Reflective diffraction grating. HCl F/G, 2

Claims (1)

[Claims] 1. An external cavity stabilized single mode laser diode having front and back reflective facets, comprising: (a) viewing the light emitted by said laser diode into a parallel bundle of light beams of different wavelengths; (b) a holographic planar transparent diffraction grating for dispersing said rays according to their wavelength; and (c) a collimating objective lens for reflecting only one of said rays onto itself. and a beam splitting mirror adapted to return the beam to the diode through the diffraction grating and the collimating lens;
The lens focuses the one beam onto the internal emitting front facet of the laser diode and excludes all other reflected beams, so that the laser diode focuses the one beam onto a single longitudinal mode corresponding to the wavelength of the one beam. A laser diode characterized in that it is operated by. 2. The laser diode of claim 1 including an anti-reflection coating on the front facet to substantially reduce the reflectivity of the front facet. 3. The laser diode of claim 2 including a reflective coating on the back facet to increase the reflectivity of the back facet. 4. A laser diode according to claim 1, wherein the beam splitting mirror comprises two prisms, with a partially reflective surface interposed between them. 5. The laser diode according to claim 1, wherein the beam splitting mirror is totally reflective. 6. The laser diode according to claim 1, wherein the diffraction grating and the beam splitting mirror constitute a reflective diffraction grating. 7. A laser diode as claimed in claim 1, including means for adjusting the angle of incidence of the mirror to tune the device. 8. A laser diode as claimed in claim 1 including means for adjusting the angle of incidence of the grating to tune the device. 9. The laser diode of claim 1 including means for adjusting the angle of incidence of both the mirror and the grating to tune the wavelength of the device. 10. A method of preventing mode hopping in a laser diode to enhance its use as an image scanning beam source in a raster output scanner, comprising: (a) collimating the light emitted by the laser diode into a parallel beam; each of said rays having a different wavelength, and further comprising: (b) dispersing said rays according to the wavelength of the individual rays; and (c) one of said dispersed rays. reflecting only one beam onto itself and back into the gain medium of the laser diode, causing the laser diode to operate in a single mode corresponding to the wavelength of the beam. 11. Anti-mode hopping according to claim 10, comprising the step of coating the lasing facet with an anti-reflection coating so that the laser diode serves only as a gain medium to render the internal cavity of the laser diode inactive. Method. 12. A method for preventing mode hopping in a laser diode to enhance its use as an image scanning beam source in a raster output scanner, comprising: (a) coating at least the front lasing facet of the diode with an antireflection material; (b) collimating the light emitted by said diode into a parallel beam; (c) dispersing said beam according to the wavelength of the individual beams; and (d) one of said beams. reflecting one beam back onto itself and onto a back lasing facet of the diode to cause the diode to operate in a single mode at the wavelength of the beam.